A 35-year-old man with no prior cardiac history presents with a 6-hour history of palpitations. He denies chest pain or dyspnea. On examination, BP is 132/82 mmHg, HR 160/min, regular. The JVP shows regular oscillations without cannon a waves. A 12-lead ECG reveals a narrow-complex tachycardia at 160/min; P waves are not clearly visible in the surface ECG. Carotid sinus massage is performed, which transiently slows the AV nodal conduction but does not terminate the arrhythmia; the tachycardia resumes at 160/min. Adenosine 6 mg IV is then administered, resulting in transient AV block and brief sinus rhythm, after which the tachycardia resumes at the same rate. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The critical finding here is that adenosine causes **transient AV block and brief sinus rhythm, followed by resumption of tachycardia at the same rate**. This pattern is pathognomonic for **atrial tachycardia (AT)**, where the arrhythmia originates in the atrium independent of the AV node. ## Why Adenosine Behavior Differs Across SVTs | SVT Type | Adenosine Effect | Mechanism | Resumption? | |----------|------------------|-----------|-------------| | **AVRT** | Abrupt termination | Blocks AV node, interrupts reentrant circuit | No resumption | | **AVNRT** | Abrupt termination | Blocks AV node, interrupts dual pathway reentry | No resumption | | **Atrial Tachycardia** | Transient AV block → brief sinus rhythm → resumption at same rate | AT focus independent of AV node; adenosine only blocks AV conduction temporarily | **Yes, resumes** | | **Atrial Flutter** | Transient AV block → flutter continues | Adenosine blocks AV node but flutter persists; sawtooth pattern visible | Continues (not sinus) | ## Distinguishing AT from Reentrant SVTs 1. **Adenosine response pattern:** In AVRT and AVNRT, adenosine terminates the arrhythmia because it blocks the AV node, which is part of the reentrant circuit. In AT, the tachycardia originates in the atrium and is **independent of the AV node**; adenosine only transiently blocks AV conduction, allowing a brief window of sinus rhythm before the atrial focus resumes firing. 2. **Absence of cannon a waves:** AT does not produce prominent cannon a waves because there is AV dissociation or normal sequential AV conduction, not simultaneous atrial and ventricular contraction. 3. **Carotid massage effect:** Carotid massage slows AV conduction but does not terminate AT (unlike AVRT/AVNRT, where it may terminate the arrhythmia by blocking the AV node). Here, carotid massage slowed conduction but did not terminate the tachycardia — consistent with AT. **High-Yield:** **Atrial tachycardia is the only SVT where adenosine causes transient AV block followed by resumption of the same tachycardia.** This is a high-yield distinguishing feature for NEET PG. **Clinical Pearl:** Atrial tachycardia can be: - **Automatic** (abnormal automaticity of atrial myocytes; seen in digitalis toxicity, hyperthyroidism, catecholamine excess) - **Triggered** (early afterdepolarizations or delayed afterdepolarizations) - **Reentrant** (micro-reentry within atrial tissue) In young, healthy patients without prior cardiac disease (as in this case), automatic or triggered AT is most common. ## Mermaid Diagram: SVT Response to Adenosine ```mermaid flowchart TD A[Narrow Complex Tachycardia]:::outcome --> B[Adenosine 6 mg IV]:::action B --> C{Response?}:::decision C -->|Abrupt termination| D{Reentrant circuit?}:::decision D -->|Yes, AV node involved| E[AVRT or AVNRT]:::outcome C -->|Transient AV block, then resumption at same rate| F[Atrial Tachycardia]:::outcome C -->|AV block, but flutter pattern persists| G[Atrial Flutter]:::outcome C -->|No response| H[Sinus Tachycardia or Sinus Node Reentry]:::outcome ``` ## Why This Is NOT AVRT or AVNRT - **AVRT/AVNRT:** Both depend on the AV node for the reentrant circuit. Adenosine blocks the AV node → circuit interrupted → arrhythmia terminates and does NOT resume. - **This case:** Adenosine caused transient AV block but the tachycardia resumed at the same rate → the arrhythmia is independent of the AV node → **AT**. [cite:Harrison 21e Ch 297]